The present invention relates to an X-ray imaging apparatus using a CCD sensor, and particularly to an X-ray imaging apparatus which displays an image on a CRT or the like by means of digital processing and without using a film, and a recording medium constituting the apparatus. The X-ray imaging apparatus of the present invention is useful for a medical or dental diagnostic apparatus or an industrial nondestructive inspection apparatus.
A conventional X-ray imaging apparatus uses an X-ray film in order to recognize the internal state of an object of diagnosis, inspection, or the like in the form of an image. Recently, an X-ray imaging apparatus using a CCD sensor has been developed for the purposes such as those of shortening the development time, facilitating the data storage, and preventing data from deteriorating. Such an X-ray imaging apparatus uses a method in which the CCD sensor directly receives X-rays and digitized data are displayed on a CRT. The conventional X-ray imaging apparatus is configured so that the CCD sensor receives light from a material which converts X-rays transmitted through an object part e.g. a diseased part to be examined into light, and image data which are digitized pixel by pixel are displayed on an image display device.
In a conventional X-ray imaging apparatus which uses a CCD sensor in this way, the trouble of development can be decreased as compared with the method used in a former apparatus in which an image is formed by exposing an X-ray film to X-rays, thereby shortening the time required for diagnosis of a diseased part.
The conventional X-ray imaging apparatus using a CCD sensor has advantages that image data are digitized and hence the data are not deteriorated, and that the image data can be collectively stored into a recording medium and hence the space for storing the data can be reduced.
In such a conventional X-ray imaging apparatus, the brightness, the contrast, and the magnification degree of the diseased part on the screen of the image display device can be freely adjusted. Consequently, the apparatus serves as effective auxiliary means for diagnosis or inspection. In the medical and dental fields, particularly, digitization of image data allows such an X-ray imaging apparatus to be useful as an auxiliary to diagnosis. When used in an industrial nondestructive inspection apparatus, such a conventional X-ray imaging apparatus can improve the inspection efficiency and enhance the inspection accuracy.
A CCD sensor used in a conventional X-ray imaging apparatus is configured in the same manner as that used in a video camera or the like, or has a configuration in which the CCD sensor receives visible light, the visible light is converted in real time into an analog signal, and the analog signal is then output. Unlike the conventional CCD sensor used in the video camera or the like, however, a CCD sensor used in an X-ray imaging apparatus is provided with a fluorescent member (a scintillator made of, for example, Gd.sub.2 O.sub.2 S) which converts X-rays into visible light, on the surface of the CCD. Also an X-ray imaging apparatus of another kind in which, in place of such a fluorescent member, a cadmium telluride detecting element (CdTe detecting element) for converting X-rays into electric charges is connected to the surface of the CCD for each pixel is used.
Usually, the inspector observes a still X-ray image obtained by an X-ray imaging apparatus. In a conventional X-ray imaging apparatus, therefore, an analog signal from a CCD sensor is converted into a digital signal by an A/D converter at the timing when X-rays are irradiated, and the digital signal is once accumulated in a memory. Furthermore, an image display device is configured so that the digitized image data are displayed as a still image on a CRT or the like.
In order to know the timing when X-rays are irradiated, a conventional X-ray imaging apparatus which uses a CCD sensor as described above must receive trigger signals notifying the start and end of exposure from an X-ray exposure apparatus.
Next, an example of a conventional X-ray imaging apparatus using a CCD sensor will be described with reference to the accompanying drawings. FIG. 23 is a block diagram showing the whole configuration of a conventional X-ray imaging apparatus using a CCD sensor, and FIG. 24 shows a flow of a procedure conducted from the start of exposure to the display of an image in the conventional X-ray imaging apparatus.
Referring to FIG. 23, an X-ray exposure apparatus 210 irradiates a diseased part which is an exposure object, and an X-ray detection unit 201 receives X-rays 211 which have been transmitted through the exposure object. The X-ray detection unit 201 comprises a CCD element 212, an A/D converter 215, and a CCD driver 216, and supplies a digital image signal 217 corresponding to the X-rays 211, to an accumulation unit 205. The accumulation unit 205 to which the digital image signal 217 is supplied comprises an accumulation value calculation circuit 223 and a frame memory 225.
At the same time when the X-rays 211 are irradiated, the X-ray detection unit 201 outputs an X-ray exposure start trigger signal 241 to an accumulation start circuit 221 of an accumulation start unit 204. Simultaneously with the end of exposure of the X-rays 211, the X-ray detection unit 201 outputs an X-ray exposure end trigger signal 242 to an accumulation stop circuit 230 of an accumulation stop unit 208. At this time, the accumulation stop circuit 230 supplies an accumulation stop instruction signal 231 to the accumulation value calculation circuit 223, and at the same time a display instruction flag 232 to a display instruction circuit 233 of a display instruction unit 209. Upon receiving the display instruction flag 232, the display instruction circuit 233 outputs a display instruction signal 234 to a CPU 235 of an image display unit 238. Upon receiving the display instruction signal 234, the CPU 235 acquires digital image data for display 226 from the frame memory 225, and displays the data on an image display device 237 such as a CRT. As required, digital image data for storage 239 output from the CPU 235 are stored as image data into a storage medium 240.
In FIG. 23, the arrows indicate the X-rays and the flows of the signals, the numeral 213 indicates an analog image signal, the numeral 214 indicates a CCD driving signal, the numeral 222 indicates an accumulation start instruction signal, the numeral 224 indicates an accumulated digital image signal, and the numeral 236 indicates an image display signal.
Referring to FIG. 23, when the X-ray exposure apparatus 210 irradiates the CCD element 212 with the X-rays 211, the CCD element 212 outputs the analog image signal 213 corresponding to an image of the exposure object, to the A/D converter 215. The A/D converter 215 to which the analog image signal 213 is supplied outputs the digital image signal 217.
Simultaneously with exposure of the X-rays 211, the X-ray exposure apparatus 210 outputs the X-ray exposure start trigger signal 241 to the accumulation start circuit 221. The accumulation start circuit 221 always monitors the input state of the X-ray exposure trigger signal 241.
FIG. 24 shows the flow of the procedure conducted from the start of exposure of X-rays to the display of an image in the thus configured X-ray imaging apparatus of the prior art. The accumulation start circuit 221 conducts the process of STEP 2 of the flow shown in FIG. 24. When the X-ray exposure start trigger signal 241 is not input, the accumulation start circuit 221 does not conduct accumulation of image data in STEP 3.
By contrast, when the X-ray exposure start trigger signal 241 is supplied to the accumulation start circuit 221 of the accumulation start unit 204, accumulation of the image data is started in STEP 4. At this time, the accumulation start circuit 221 of the accumulation start unit 204 outputs the accumulation start instruction signal 222 to the accumulation value calculation circuit 223. When the accumulation start instruction signal 222 is input to the accumulation value calculation circuit 223, the circuit converts the digital image signal 217 which is thereafter periodically transmitted, into the accumulated digital image signal 224, and stores the converted signal into the frame memory 225.
Next, the analog image signal 213 which is an output signal from the CCD sensor having the CCD element, and the method of the A/D conversion of the analog image signal 213 in the A/D converter 215 will be briefly described with reference to FIGS. 25 and 26.
FIG. 25 is a view conceptually showing the configuration of pixels in the CCD sensor, and FIG. 26 is a view conceptually showing the method of A/D converting the analog image signal 213 in the CCD sensor.
As shown in FIG. 25, the CCD element sensor has pixels arranged in a predetermined number in both the vertical and lateral directions. As shown in FIG. 26, the CCD sensor always outputs at a constant period the analog image signal containing image data of all the pixels of the CCD sensor. The image data of all the pixels are A/D converted at each period by the A/D converter 215, thereby producing the digital image signal 217. The digital image signal 217 is accumulated and then stored into the frame memory 225. The CPU 235 acquires the digital image data for display 226 which are the stored digital image data, from the frame memory 225, and the acquired image data are displayed on the image display device 237 such as a CRT.
Next, at the same time when exposure of the X-rays 211 is ended, the X-ray exposure apparatus 210 outputs the exposure end trigger signal 242 to the accumulation stop circuit 230. The accumulation stop circuit 230 conducts the process of STEP 5 of the flow shown in FIG. 24, and always monitors the input of the exposure end trigger signal 242. When the exposure end trigger signal 242 is not input, accumulation of image data is continued in STEP 6.
By contrast, when the exposure end trigger signal 242 is supplied to the accumulation stop circuit 230, accumulation of the image data is stopped in STEP 7 of FIG. 24. At this time, the accumulation stop circuit 230 outputs the accumulation stop instruction signal 231 to the accumulation value calculation circuit 223, and at the same time the display instruction flag 232 to the display instruction circuit 233.
When the accumulation stop instruction signal 231 is supplied to the accumulation value calculation circuit 223, the processes of accumulating the digital image signal 217 and storing the accumulated signal into the frame memory 225 are stopped. When the display instruction flag 232 is supplied to the display instruction circuit 233, the display instruction circuit 233 outputs the display instruction signal 234 to the CPU 235. Upon receiving the display instruction signal 234, the CPU 235 acquires the digital image data for display 226 from the frame memory 225, and displays the data on the image display device 237 such as a CRT. As required, the image data are stored into the storage medium 240.
In the conventional X-ray imaging apparatus using a CCD sensor, the processes of displaying an image and storing image data are conducted by the method described above.
X-rays have a unique characteristic that, although with a low probability, X-rays can be transmitted through an irradiated portion through which they should originally be transmitted. X-rays transmitted through such a portion may result in appearing as a white dot on a screen. When an image is directly produced from image data of a digital signal obtained by A/D converting an analog image signal from a CCD sensor, therefore, several white dots appear in the whole of the image. When such image data are used, there arises a problem in that diagnosis cannot be performed with high reliability.
In a conventional X-ray imaging apparatus, the input and output operations for an X-ray exposure apparatus and an image display device are cumbersome, and hence the X-ray exposure apparatus and the image display device must be integrally formed in order to attain higher operability. Consequently, an existing X-ray imaging apparatus using an X-ray film cannot be adapted for the use of a CCD sensor, thereby producing an economic problem that such an apparatus is wasted.
Furthermore, an optimum image quality must rely on the control of the output of an X-ray exposure apparatus. Therefore, a conventional X-ray imaging apparatus has also a problem in that it is difficult to adjust the image quality.
In a conventional X-ray imaging apparatus, the intensity of an X-ray is displayed as it is as an image in the term of brightness. Such an X-ray image is not sufficient as an image which the inspector observes, and must be subjected to a process such as density correction because, in order to obtain an image of the visually highest level, a dark portion is to be displayed more finely than a bright portion and the brightness of the displayed image is to have a range as wide as possible. However, the work of adjusting density and the like while observing an image requires the inspector to have technical skills. Practically, it is difficult to process many images for a short time. This makes a large problem in observation of an X-ray image.
Since image data consist of digital values, the range of the data is finite. When the exposure amount of X-rays exceeds the range or is too large or small, density cannot be adjusted, thereby producing a problem in that an image sufficient for diagnosis cannot be obtained.
A CCD sensor used in an X-ray imaging apparatus is superior in sensitivity to a film, and has superiority in certainty that a sufficient image can be obtained by a small exposure amount, and accuracy. However, a CCD sensor has a problem in that it is very difficult to set conditions for adjusting the exposure time and the exposure distance so as to conform to the sensitivity of the CCD sensor. In the dental field, for example, the transmittance of X-rays is varied depending on the kind of a tooth, i.e., an anterior tooth or a molar tooth, or a tooth of an infant or that of an adult, and hence the exposure amount of X-rays must be set in a different manner. In a conventional X-ray imaging apparatus using a CCD sensor, therefore, test exposure must be repeated until an image is obtained under optimum conditions. In other words, useless exposure must be conducted.